Inverter sizing

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Inverter sizing

Postby mblitch1 » Thu Sep 21, 2006 12:07 pm

In my teardrop, I currently have a pigtail going from shore power to a GFI that is upstream of several outlets around the camper (in the kitchen area and inside for a fan and such). I want to get an inverter to run some other things, specifically a stand up fan for outside use, microwave for quick heating, and maybe a blender for tailgating events. I have some 8ga wire going from the battery to the back where I plan to put the inverter. I plan to add a second pigtail to the GFI to plug into the inverter to power the outlets when away from shore power.

I've been looking at various inverters and had a few questions that I could not find in an FAQs. First off, I want to get a power point plug to also use the inverter in the car to power a laptop at times. Most high wattage inverters do not have one, but low wattage ones do. I assume this is to prevent people from trying to draw too much power through the low amp rated dashboard wiring. If I am sure to limit the wattage usage, am I incorrect in thinking I can just as readily use a high wattage inverter? I wouldn't think the higher rated inverter arbitrarily draws more amps when the powered device (pretty much will be just a laptop and maybe small (AA/AAA) battery charger) doesn't need it.

Any particular recommendations regarding inverters brands/models. I figured something like the Xantrex 700W (for about $50 on amazon) would suffice. I would like something that will auto shut down if the battery gets too low (I already have the voltage / state of charge chart so I can manually check). I'll have to, of course, be sure that the pigtail is removed from the inverter whenever plugged into shore power.

Also, to reduce sparking when connecting the leads, I planned to use a simple wall switch in an outdoor rated box next to the battery to act as a master switch. I planned to just cut the positive 8ga wire and attach it to the switch and cut it off when attaching the leads or when away from the camper. Any concerns regarding this kind of set up that I may not have assumed yet?
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Postby sjptak » Thu Sep 21, 2006 4:19 pm

I've got a few comments based on my own experience. The 700W inverter will be too small. The smallest micro that I've seen will draw 800W. It works fine on my 1200W continuous 2400W peak inverter.

On the 120V side, I plugged a pigtail that led to a GFI. I then plugged my trailer into that. I made sure all grounds were good. The inverter was grounded to the frame and the trailer skin, as was the GFI. I fried three GFI's before I gave that idea up. I replaced the GFI with a standard grounded outlet. Everything has been fine since.

You hit the nail on the head when you figured in the cheesy dash wiring. Everything I read says the inverter (bigger ones anyways) should be located as close as possible to the battery(ies). Extensions on the 120V output are OK. Use a heavy gauge wire. I have 2 6V batts with 6Gauge travelling 4 feet. I've camped 5 nights and with a bit of conservation have never run down the batts too far. Of course I don't leave the inverter on when not in use.

I'd use a marine battery cut off switch if I wanted to be able to cut the whole thing down.

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Postby Q » Thu Sep 21, 2006 8:07 pm

I would use a minimum 1000 watt inverter to power a microwave. It needs to be as close a possible to the battery with large size cables. Get a seperate smaller inverter to plug into the cig lighter socket.

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Postby asianflava » Thu Sep 21, 2006 8:52 pm

I think my microwave is stronger than I thought (said 1000W on the box). I'm using a 1200W inverter and it doesn't run it. I get one run then it trips out the inverter n the next run. I think I'll step up to a bigger inveter.
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Postby Q » Thu Sep 21, 2006 10:57 pm

Any idea what the battery voltage is when the inverter kicks off? The inverter probably doesn't have enough capacity but the battery or cables could also be a problem.

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Postby asianflava » Fri Sep 22, 2006 12:20 am

Q wrote:Any idea what the battery voltage is when the inverter kicks off? The inverter probably doesn't have enough capacity but the battery or cables could also be a problem.

Q


I doubt it's the battery cables. I'm running 4ga from the battery to the inverter plus it's only about 18inches long.
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Postby Dale M. » Fri Sep 22, 2006 11:12 am

asianflava wrote:
Q wrote:Any idea what the battery voltage is when the inverter kicks off? The inverter probably doesn't have enough capacity but the battery or cables could also be a problem.

Q


I doubt it's the battery cables. I'm running 4ga from the battery to the inverter plus it's only about 18inches long.


Check voltage drop across battery during inverter/microwave "run"...

Also Your 1000watter may be overheating and going into thermal shutdown..

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Postby madjack » Fri Sep 22, 2006 11:25 am

due to inherent inefficiencies in an iverter, I would think that a 1500 watt unit would be the least needed for running a hi-power appliance...rember power conversion between 120vac and 12vdc is a factor of 10...that 10 amp 120v appliance will pull 100A outta the battery, which will kill a battery almost immediately...unless you have an abundance of 12v power, this is not a good/feasible idea.......
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Postby Dale M. » Fri Sep 22, 2006 1:54 pm

madjack wrote:due to inherent inefficiencies in an iverter, I would think that a 1500 watt unit would be the least needed for running a hi-power appliance...rember power conversion between 120vac and 12vdc is a factor of 10...that 10 amp 120v appliance will pull 100A outta the battery, which will kill a battery almost immediately...unless you have an abundance of 12v power, this is not a good/feasible idea.......
madjack 8)


Parallel batteries, but then there is the weight factor...

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Postby asianflava » Fri Sep 22, 2006 2:27 pm

Dale M. wrote:Also Your 1000watter may be overheating and going into thermal shutdown..


I have a 1200W inverter and a 1000W nuker

madjack wrote:..rember power conversion between 120vac and 12vdc is a factor of 10...that 10 amp 120v appliance will pull 100A outta the battery, which will kill a battery almost immediately8)


I figure the mine pulls around 40A-50A because I have 50A fuse inline that hasn't blown. I had to buy a bunch of fuses with different ratings then step it up till it didn't blow. I figured that the nuke would only be running 30-60 seconds at a time. I rigged mine this way in case I had to dry camp. So far I never have, so I haven't used it in battery mode.
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Postby Dave Bob » Fri Sep 22, 2006 4:00 pm

The Killawatt is a great tool to see how much power an appliance is using. I have a 700 watt nuker that pull between 1150 to 1200 watts. I don't think the "cooking" wattage is any where close to how much the unit actually uses.

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Postby madjack » Fri Sep 22, 2006 4:10 pm

Dave Bob wrote:The Killawatt is a great tool to see how much power an appliance is using. I have a 700 watt nuker that pull between 1150 to 1200 watts. I don't think the "cooking" wattage is any where close to how much the unit actually uses.

Dave


DB, that is a neat litle tool...affordable as well :thumbsup: .............................................. 8)
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Postby mblitch1 » Tue Sep 26, 2006 12:44 pm

I picked up a 'Kill-A-Watt' a while ago and also find it useful for figuring out how much power the system can take. I decided to pick up a 1500W inverter that was at Sam's Club and will attach it directly to the battery.

Assuming a start up of 2000 watts, then am I correct in figuring that the draw would be ~166 amps? So should I get a high amp fuse such as a 200 rated one? They seem decently expensive, but if it saves the 100$ inverter and any other electronics, then it might be worthwhile.
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Postby bledsoe3 » Wed Sep 27, 2006 1:06 am

I don't think you want 166 amps running thru your tear. :shock:
One of the more electrically gifted will chime in I'm sure. You should never need more than a 20 amp breaker in your tear.
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Postby Leon » Wed Sep 27, 2006 1:29 am

bledsoe3 wrote:I don't think you want 166 amps running thru your tear. :shock:
One of the more electrically gifted will chime in I'm sure. You should never need more than a 20 amp breaker in your tear.

we may be talking two different things here. the 166 amps is at 12 volts, and you're talking about what should be a practical limit on what would be needed for 120 volts. If you are talking about 20 amps at 12 volts, that's lighting and a TV which won't exceed a couple hundred watts, but when you start getting into inverters the wattage might be more which will mean more amps. A 2000 watt inverter will draw the 166 amps at 12 volts, and put out 16 amps at 120 volts(theoretical values - will change slightly because of battery voltage, efficiency, etc)
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